For many years, thermocouple junctions have been employed in the measurement of temperatures in industrial processes and in machines having constituent parts whose temperatures are important either because of their effect on other constituent parts or because of the permitted operating temperature range of the parts themselves. It will be understood that a thermocouple junction for measuring temperature comprises at least two dissimilar materials which are intimately connected together and which, by reason of their dissimilarity, generate a thermoelectric voltage that is a function of the temperature of the junction. Such thermocouple junctions are commercially available in very small sizes so that they may be employed in the measurement of temperatures of extremely small objects such as the pins or "legs" of semiconductor devices and of other "chips" and integrated circuits that may have a great many legs disposed very closely together around the edges of the chip. For instance, a microprocessor chip, such as those marketed by Intel, Motorola, and Texas Instruments, may have as many as 50 legs per inch around its periphery.
Although thermocouples are available in sub-miniature sizes having dimensions which are very small even when each delicate thermocouple junction is surrounded by a protective sheath, there has not until now been a satisfactory commercially-available means for temporarily holding the thermocouple junction or its sheath in intimate thermal contact with a device or other extremely small object whose temperature is to be sensed and measured. Furthermore, there has not been a satisfactory way to attach temporarily a probe containing a thermocouple Junction to a very small object whose temperature is to be sensed and measured, while permitting the probe to be easily released from the object without damaging or moving it.
In the "surface-mount" method of attaching electronic components to a circuit board, the components are carefully placed upon "pads" of solder paste that have been deposited on corresponding pads on the circuit board through a mask so as to constitute a "foundation" for the electronic components which are to be fastened to the board without having their pins or legs extend through holes in the board. Each electronic component is then precisely positioned on the board so that each of its legs rests in a pad of solder paste on the surface of the board. Until each pad of solder paste has been fused and in turn solidified, the electronic component is not so firmly adhered to the board that its position on the board is permanently fixed. In accordance with the practices of modern surface-mount technology, the pads of solder paste are fused by placing the board and its associated electronic components in a "reflow oven" filled with air or inert gas which is sufficiently hot to fuse the pads of solder paste. Upon leaving the oven, the temperature of the board and its associated components falls to a level such that the solder hardens, thereby retaining the components permanently fixed in place on the surface of the board.
During the reflow process, it is usually necessary to determine and maintain the temperatures of certain critical components. For instance, it is generally necessary that the temperature of the "coldest" component on the board be at least sufficient to ensure the prompt and thorough fusing of all the solder pads on which it is mounted. Moreover, it is vital that, during the soldering operation, the temperature of the "hottest" component not attain a level that would be harmful to the semiconductor material or to the proper distribution of the impurities therein, as set forth in the operating specifications promulgated by the manufacturer of the component. For these purposes, it may not be sufficient to sense and measure the temperature of the component as a whole, on the assumption that the temperature of the component is uniform or that there is no particular portion thereof which has a critical threshold temperature of damage that is different from the threshold temperature of damage in the remainder of the component. On the contrary, it is frequently necessary, or at least desirable, to be able to sense the temperature of a certain small portion of the component, for example a leg or pin connected to that portion. As aforementioned, there has not heretofore been a satisfactory commercially-available means for removably positioning a thermocouple junction in close thermal contact with a very small surface area of a component whose temperature is of particular importance.
Reference has been made to the reflow-soldering processes that are now widely employed in the surface mounting of electronic components on circuit boards. In the reflow processes, the circuit board with its associated components is passed through an oven containing extremely hot air or inert gas. Of course, the use of inert gas is favored in order to prevent oxidation of copper conductors in the circuit board or of the solder itself in such a way as to impair the electrical conductivity of a soldered joint.
Just as in the case of surface mounting of electronic components on a circuit board, the same concerns for temperature measurement with high resolution prevail if solder is to be removed from a circuit board for the re-arrangement of electronic components or substitution of new ones. In either case, the circuit board and the components to be mounted or removed and any necessary temperature-measuring means are likely to be immersed in very hot gas which may in some cases be in rapid motion. It remains important for the thermocouple probe and its junction of dissimilar materials to measure the temperature of a particular portion of a particular component, rather than that of the mass of ambient gas.
Finally, provision must be made for supporting the weight of the probe while it is in operation. Although the thermocouple junction itself may be very small in size and have a mass that is almost negligible, the structure for supporting the thermocouple junction and protecting it against its environment during temperature sensing and measurement may be quite substantial. In certain instances, it may be feasible for the object whose temperature is being sensed to support part of the weight of the thermocouple probe. In other instances, the weight of the thermocouple probe and its associated structure must be externally borne. For instance, if there is any possibility of deformation of the circuit board during the heating and cooling steps of the reflow-soldering process, the probe and its thermocouple junction should not be carried by any portion of the circuit board that might bend or buckle, thereby causing the thermocouple junction to be mispositioned with respect to the portion of the component or other object whose temperature is to be sensed and measured.
Of course, the prior art in the field of thermocouple probes is very highly developed. An example of such prior art is U.S. Pat. No. 3,539,400-Pustell, issued Nov. 10, 1970. That patent discloses a "composite support" for a thermocouple probe for operation in a high-temperature environment. Pustell shows a sheathed thermocouple junction, part of which is in turn surrounded by another sheath that is designed to be particularly resistant to the environment. However, the space between the two aforementioned sheaths is packed tightly with a refractory material for the purpose of "substantially eliminating all voids between sheaths, . . . " Moreover, Pustell shows no means for causing the thermocouple probe to be temporarily attachable to a device whose temperature is to be measured, because the Pustell thermocouple probe was designed primarily for sensing the temperature of hot oases flowing through an aircraft gas turbine. Further, Pustell provides no means for exerting a force to maintain the thermocouple probe in close contact with a device or other solid object. Still further Pustell's disclosure does not include any means for supporting the mass of the thermocouple probe other than through the thermocouple leads and their sheaths.
A thermocouple heat sensor which does include means for physical support is illustrated in U.S. Pat. No. 4,626,643-Minet, issued on Dec. 2, 1986. The Minet patent discloses a heat sensor for measuring the temperature of a product heated in a microwave oven. Provision is made for shielding a thermocouple junction from the microwave energy in the oven, rather than from high-temperature gases. Between the shielding material and the thermocouple junction and its electrical leads there are solid spacers which would be effective conductors of heat from the environment to the thermocouple junction. Moreover, the Minet disclosure, like the Pustell disclosure mentioned above, contemplates relative motion in an axial direction between the shielding material, on the one hand, and the thermocouple junction and its electrical leads on the other hand. The Pustell patent provides, between lines 47 and 50 of Column 3, that the "inner material sheath 20 is permitted to float or slip between the environmental resistant material sheath 22 and thermocouple probe sheath 18 when the thermocouple assembly is subjected to vibration, thermal shock, or differential expansion." Similarly, Minet provides separate spring loading which effectively ensures that the microwave-protective material and the thermocouple junction and leads are advanced at different rates. There is no means for assuring, or even permitting, rigidity of the shielding material with respect to the thermocouple junction in an axial direction. Moreover, Minet discloses a "thermally conductive pad" at the end of his microwave-protective material which virtually guarantees the conduction of heat from ambient air to the thermocouple junction.
Although Minet does provide structural means for supporting the heat sensor, he provides no means for precisely positioning it with respect to a very small component or object whose temperature is to be sensed and measured. The Minet sensor would not be at all suitable for measuring the temperature of the leg of an electronic device positioned on a circuit board, for instance. Accordingly, it appears that the prior art, as represented by the aforementioned patent references, is grossly deficient in disclosing satisfactory means for sensing and measuring the temperature of small components or objects in a hot environment without distortion by reason of the presence of the hot environment.